70 



GYMNOGRAMMOID FERNS 



[CH. 



leaf-gaps are elongated, so that in transverse section it often appears as two 

 separate straps. The leaf-trace departs as a simple strand. The venation of 

 the leaves is an open dichopodium that 

 spreads fan-like, its ends extending into 

 the recurved margins of the leaves (see 

 Thompson, 647). 



The sporangia are distributed along the 

 veins: but some are inserted on the general 

 surface of the pinna between them : this 

 may be held as an advance upon the usual 

 Gymnogrammoid type towards that which 

 is described as Acrostichoid (Fig. 629). The 

 various ages of sporangia are intermixed. 

 The sporangia themselves are usually 

 pear-shaped but lop-sided, with a three- 

 rowed stalk. The annulus is vertical, and 

 interrupted at the insertion of the stalk: 

 it has about twenty indurated cells, but 

 these and the stomium are both variable 

 as regards detail. The spores are tetra- 

 hedral. Spore-counts range from fifty-six 



to seventy-two. The latter figure, which choid." (After Di'j. McL. Thompson.) 

 was found in large sporangia with an irregular annulus, is most unusual in 

 Ferns with a mixed sorus. All these features together suggest a transitional 

 state, where the sporangial structure has not become fully standardised. 

 They point out Jamesonia as a Fern combining characters that are primitive 

 with indications of advance (see Thompson, 647). 



Ceratopteris Brongn. 



Ceratopteris thalictroides (L.) Brongn. is usually placed as the sole repre- 

 sentative of the Parkeriaceae. The story of its early vicissitudes of classification 

 is told in H.ooker's Species Filiciun, Vol. II, p. 236. This monotypic Fern grows 

 throughout the tropics rooted in mud or shallow water, or sometimes floating. 

 The short upright shoot bears spirally arranged sappy leaves with reticulate 

 venation. The first of these are sterile with broad irregular lobes, the later 

 leaves are fertile with narrower lobes, their recurved margins protecting the 

 lower surface on which the large isolated sporangia are borne. Scales are 

 present on the axis and leaf-bases. 



The aquatic habit and the very sappy nature of its tissues have made the 

 study of the vascular system difficult. The account of it here given is based 

 upon the MS. notes of the late Prof. D. T. Gwynne-Vaughan. The axis after 

 initial protostely widens to produce a typical radial dictyostele, with added 



Fig. 629. Fertile pinna oi Jamesonia verii- 

 calis Ivze., showing the venation, and inser- 

 tion of the sporangia, the latter is "Acrosti- 



